Recently, aromatic cyclic compounds are drawing attention its application/development potential as a high-performance material or functional material based on the properties arising from the cyclic structure (e.g., as a compound having clathrating capacity and as a effective monomer in the synthesis of a high-molecular-weight linear polymer by ring-opening polymerization) as well as for its structure-derived specificity. Cyclic poly (phenylene ether ether ketone)s are also noteworthy in the same manner since they belong to the category of aromatic cyclic compounds.
As a method of synthesizing a cyclic poly (phenylene ether ether ketone), there is reported, for example, a method in which a linear poly (phenylene ether ether ketone) oligomer having a hydroxyl group at both terminals and a linear poly (phenylene ether ether ketone) oligomer having a fluorine group at both terminals are reacted as shown in the following reaction formula (for example, see Non-patent Document 1).

In this method, since long-chain oligomers are used as starting materials, the resulting poly (phenylene ether ether ketone) mixture is composed of cyclic poly (phenylene ether ether ketone) having a repeating number (m) of 3 and/or 6; therefore, this method can yield only a cyclic poly (phenylene ether ether ketone) having a melting point higher than 270° C. More specifically, it is described that the cyclic poly (phenylene ether ether ketone)s obtained from these linear oligomers shown in the above reaction formula (an oligomer having a hydroxyl group at both terminals, which is constituted by 4 benzene ring component units; and an oligomer having a fluorine group at both terminals, which is constituted by 5 benzene ring component units) are composed of only a cyclic trimer (m=3) and cyclic hexamer (m=6), which are cyclic poly (phenylene ether ether ketone)s having a melting point of 366° C. and 324° C., respectively. Further, Non-patent Document 1 also offers descriptions regarding the synthesis of a poly (phenylene ether ether ketone) by ring-opening polymerization of a cyclic poly (phenylene ether ether ketone); however, the cyclic poly (phenylene ether ether ketone) used therein is the one having a high melting point obtained in the above-described method. Moreover, the ring-opening polymerization is performed only in a temperature range of not lower than 340° C., that is, a temperature range which is not lower than the melting point of the poly (phenylene ether ether ketone), and there is no description at all with regard to ring-opening polymerization at a temperature not lower than the melting point of the poly (phenylene ether ether ketone).
Further, the same authors also discloses a method of producing a cyclic poly (phenylene ether ether ketone) by reacting a linear poly (phenylene ether ether ketone) oligomer having a hydroxyl group at both terminals with 4,4′-difluorobenzophenone as shown in the following reaction formula (for example, see Non-patent Document 2).

It is described that the cyclic poly (phenylene ether ether ketone) obtained by this method is a mononuclear compound of a cyclic dimer (m=2) and has a melting point of not lower than 440° C. In this manner, the use of a linear poly (phenylene ether ether ketone) oligomer as a material for synthesizing a cyclic poly (phenylene ether ether ketone) can be considered as a significant method for the purpose of obtaining a cyclic poly (phenylene ether ether ketone) having a desired repeating number (m) in a high purity; however, by this method, it is difficult to produce the cyclic poly (phenylene ether ether ketone) composition of the present invention, which is characterized by being a mixture composed of cyclic poly (phenylene ether ether ketone)s having different repeating numbers (m) and having a melting point of not higher than 270° C. Further, in the synthesis of a cyclic poly (phenylene ether ether ketone) according to Non-patent Documents 1 and 2, since the reaction is performed under pseudo-high-dilution conditions, although the selectivity for the generation of cyclic poly (phenylene ether ether ketone) is high, an ultra-dilute condition must be maintained, so that the reaction requires an extremely long time. In addition, it is also required to perform the step of separately preparing the oligomer having a hydroxyl group at both terminals and the oligomer having a fluorine group at both terminals that are used as the materials for the synthesis of cyclic poly (phenylene ether ether ketone). Therefore, it is difficult to say that the methods according to Non-patent Documents 1 and 2 are industrially applicable production methods of a cyclic poly (phenylene ether ether ketone).
Further, there is also reported a method using an aromatic imine compound as a starting material for producing a cyclic poly (phenylene ether ether ketone) (for example, see Non-patent Document 3). Non-patent Document 3 discloses a method in which a cyclic poly (phenylene ether ether ketimine) is prepared from N-phenyl(4,4′-difluorodiphenyl) ketimine and hydroquinone as shown in the following reaction formula and the resulting cyclic poly (phenylene ether ether ketimine) is then hydrolyzed in an acidic condition to obtain a cyclic poly (phenylene ether ether ketone).

Since an aromatic ketimine compound generally has a low reactivity as compared to the corresponding aromatic ketone compound and the reaction is performed in an ultra-dilute condition, even after the completion of the synthesis reaction of the cyclic poly (phenylene ether ether ketimine), a low-molecular-weight linear oligomer, which is difficult to be separated from the cyclic poly (phenylene ether ether ketimine), remains. Therefore, this method can yield only a low-purity cyclic poly (phenylene ether ether ketone) containing a large amount of impurities. In addition, in order to produce a cyclic poly (phenylene ether ether ketone) by this method, it is indispensable to perform multiple complicated reaction steps, including at least the steps of preparing an aromatic ketimine compound used as a starting material; preparing and purifying a cyclic poly (phenylene ether ether ketimine); and preparing and purifying a cyclic poly (phenylene ether ether ketone) by hydrolyzing the thus obtained cyclic poly (phenylene ether ether ketimine). Therefore, it is difficult to say that this method is an industrially applicable production method of a cyclic poly (phenylene ether ether ketone). Furthermore, although there is no description in Non-patent Document 3 with regard to the melting point of the resulting cyclic poly (phenylene ether ether ketone), since it contains a large amount of linear poly (phenylene ether ether ketone) having a high melting point as an impurity, it is believed that the cyclic poly (phenylene ether ether ketone) obtained by this method has, unlike the one according to the present invention, a high melting point. Moreover, Non-patent Document 3 offers no description at all with regard to ring-opening polymerization of the cyclic poly (phenylene ether ether ketone) obtained by this method.
Further, there is also disclosed a method of producing a cyclic poly (phenylene ether ether ketone) using a phenylene ether oligomer as a starting material (for example, see Patent Document 1).

In Patent Document 1, it is described that a cyclic poly (phenylene ether ether ketone) can be prepared in a single step by a reaction of 1,4-diphenoxybenzene in the presence of a Lewis acid. Synthesis methods of poly (phenylene ether ketone)-type compounds can be generally classified into two types: synthesis methods based on ether bond formation by aromatic nucleophilic substitution reaction and synthesis methods based on ketone bond formation by aromatic electrophilic substitution reaction, and the route of the cyclic poly (phenylene ether ether ketone) synthesis according to Patent Document 1 is included in the latter type. One of the problems of using an aromatic electrophilic substitution reaction for the synthesis of a poly (phenylene ether ether ketone) is that the regioselectivity of the reaction is low. Therefore, the cyclic poly (phenylene ether ether ketone) obtained by the method described in Patent Document 1, too, is speculated to have a low purity, containing ortho-form and meta-form in addition to the desired para-form. Moreover, Patent Document 1 offers no description at all with regard to the melting point of the resulting cyclic poly (phenylene ether ether ketone).